The present invention relates to a fluid bed process for the production of maleic acid or maleic anhydride from 4-carbon hydrocarbons using a vanadium phosphorus oxide (VPO) catalyst wherein phosphorus loss from the catalyst, which occurs during the course of the reaction, is replaced by impregnating VPO catalyst with alkyl ester of orthophosphoric acid, such as triethylphosphate (TEP) and adding the alkyl ester-impregnated catalyst to the fluid catalyst bed, thereby improving catalyst performance.
Large quantities of maleic an hydride are produced each year throughout the world, since maleic anhydride can be employed as a versatile intermediate for chemical synthesis and is often used in the production of alkyl resins. Maleic acid is a precursor to maleic anhydride and can also be used as the starting material for the production of 1,4-butanediol (BDO).
Maleic anhydride may be produced by vapor phase oxidation of n-butane in air using a fixed bed or fluid bed vanadium phosphorus oxide (VPO) catalyst.
The advantages of fluid bed hydrocarbon oxidation processes compared to fixed bed hydrocarbon oxidation processes are well known in the art, including the improvement of temperature control and heat transfer for oxidation reactions.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and combining the same with a phosphorus compound, and if desired, promoter element compounds under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to vanadium phosphorus oxide. The catalyst oxide precursor is then recovered and converted to active catalytic material before or after the suitable catalyst particles for either fixed bed or fluid bed are formed.
U.S. Pat. Nos. 3,888,886; 3,905,914; 3,931,046; 3,932,305 and 3,975,300 disclose the testing of promoted vanadium phosphorus oxide catalysts for maleic anhydride production from butane in one inch diameter fluid bed reactors. In most instances, the catalysts were prepared by forming the catalyst precursor in aqueous media (in U.S. Pat. No. 3,975,300 the precursor was formed in a paste of a vanadium compound, a phosphorus compound and an organic reducing agent), drying and thereafter grinding and sieving the precursor to a powder of about 74 to 250 microns size. This manner of preparation, however, does not obtain the attrition resistant catalyst particles preferred for successful fluid bed operation.
Commercial fluid bed catalysts are preferably microspheroidal particles within the range of about 20 to about 300 microns in average diameter, preferably having about 80% of the particles within the range of about 30 to about 80 microns in diameter. Most preferably, about 25 to about 40% of the particles have an average diameter of less than 45 microns.
U.S. Pat. No. 4,647,673, incorporated herein by reference in its entirety, discloses a process for the preparation of attrition resistant, microspheroidal fluid bed catalysts comprising the mixed oxides of vanadium and phosphorus in which a vanadium phosphorus mixed oxide catalyst precursor is densified, comminuted, formed into fluidizable particles and calcined under fluidization-type conditions.
As in other vanadium phosphate catalysts used for either fluid bed or fixed bed butane oxidation, phosphorus loss occurs with catalyst use. This loss may lead to a reduction of the catalyst's maleic anhydride yield. This loss is deleterious to plant capacity and economics of production. Therefore, methods of phosphorus addition have been developed to compensate for this phosphorus loss and hence recover part or all of the yield lost. Continuous addition of phosphorus also has the added advantage of maintaining the maleic anhydride yield at an economical and stable level.
United Kingdom Patent Specification 1,464,198 discloses the reactivation or regeneration of certain vanadium-phosphorus-oxygen catalyst complexes promoted with zirconium, hafnium, chromium, iron, lanthanum, or cerium by having the catalyst contacted during vapor-phase oxidation with an alkyl ester of orthophosphoric acid having the formula (RO)3P═O wherein R is hydrogen or C1 to C4 alkyl, at least one R being C1 to C4 alkyl.
In U.S. Pat. No. 4,701,433, Edwards discloses a process for the manufacture of maleic anhydride from butane in the presence of a vanadium-phosphorus-oxygen catalyst or a vanadium-phosphorus-oxygen-co-metal catalyst, wherein water and a phosphorus compound are added to the reaction system to reversibly deactivate a portion of the catalyst in the catalyst bed containing a reaction exotherm (hot spot) prior to the addition of the phosphorus compound, which addition of phosphorus compound moves the reaction exotherm downstream into the catalyst bed, and an improved catalyst bed is obtained when the partially deactivated catalyst in the original “hot spot” location reactivates to produce a more isothermal catalyst bed.
Although Edwards, et al. recognized the utility of a more isothermal catalyst bed to improve yield, Edwards accomplished a more isothermal catalyst bed temperature by shifting the location of the original “hot spot” to a new location and then reactivating the old location. Edwards teaches that improvement in yield can be obtained as long as there is sufficient catalyst bed into which the exotherm may migrate. Edwards failed to recognize that constant shifting of the exotherm is inherently unstable and not suitable for long-term operation.
U.S. Pat. No. 4,780,548, Edwards, et al. teaches a continuous process for the vapor-phase oxidation of a n-butane feedstock to form maleic anhydride in which n-butane is contacted in the presence of molecular oxygen or air at an hourly space velocity of about 100 to about 4000 cubic centimeters of feed per cubic centimeter of catalyst per hour with a vanadium-phosphorus-oxygen catalyst wherein the catalyst is regenerated continuously or batchwise by contacting it during the vapor phase oxidation with an alkyl ester of orthophosphoric acid having the formula (RO)3P═O wherein R is hydrogen or C1 to C4 alkyl, at least one R being C1 to C4 alkyl, wherein the amount of water added is about 1000 parts per million to about 40,000 parts per million by weight of the reactor feed gas stream and the amount of the alkyl ester added is about 0.1 parts per million to about 100,000 parts per million by weight of the reactor feed gas stream. The gaseous feed stream to the reactor will contain from about 0.2 to about 1.7 mole % of n-butane but about 0.8 to about 1.5 mole % of n-butane is satisfactory for optimum yield from the process of the invention. Edwards teaches that higher concentrations can be employed but explosive hazards may be encountered. Even though Edwards acknowledged that explosive mixtures could be employed, above 1.5-1.7 moles n-butanes, Edwards did not recognize that an isothermal reaction employing a higher concentration of n-butane would result in a yield continuously maintained at a high level for extended periods of time despite the possibility of explosive hazards.
Becker, et al., U.S. Pat. No. 4,795,818 disclosed a method for optimizing the yield of a vanadium-phosphorus catalyst during the oxidation of n-butane to maleic anhydride, wherein a volatile phosphorus compound is continuously added at a rate selected to maintain maximum yield while holding the operating temperature constant, the operating temperature being monitored preferably by the outlet gas temperature. Becker et al., indicate that the amount of phosphorus compound to be added should be sufficient to prevent decline in operating temperature. Becker, et al., fail to recognize the impact of the water added to the reaction and rely only on the addition of a volatile phosphorus compound to the process.
In U.S. Pat. No. 4,515,899, Click, et al., teach that the useful life of a vanadium-phosphorus-oxygen catalyst can be extended in fixed bed reactors by treatment with a phosphorus compound followed by steam treatment and furnish data showing movement of the exotherm further into the catalyst bed.
U.S. Pat. No. 5,117,007, incorporated herein by reference in its entirety, discloses a continuous process for the production of maleic anhydride by the partial oxidation of a hydrocarbon feedstock comprising n-butane in a concentration of at least 1.6 mole percent wherein a mixture of the feedstock and an oxidizing medium is contacted with a vanadium-phosphorus-oxygen catalyst wherein a solution of water and an alkyl ester of orthophosphoric acid is continually added to the feedstock, wherein the ratio of water to elemental phosphorus in said alkyl ester is in the range of from about 6500:1 wt. to about 50,000:1 wt., water to phosphorus, and the differences in reaction temperature throughout the entire reaction zone is less than about 45° C. (80° F.).
In the partial oxidation of n-butane to maleic anhydride, the reaction is highly exothermic and a catalyst hot spot temperature can develop with potential of a runaway oxidation reaction and consequent complete loss of product yield. Such a development of a hot spot temperature in a fixed bed reactor is potentially extremely detrimental to the progress of the oxidation reaction. The hot spot temperature readily occurs in the oxidation and is quite sensitive to variations in the concentration of feed hydrocarbon. Small increases in the concentration of the hydrocarbon feed can result in large increases in hot spot temperature and a concurrent decrease in selectivity and yield. In addition, high hot spot temperatures can shorten the useful life of the catalyst being employed. It is therefore necessary to avoid the development of an excessively high hot spot temperature and to maintain an isothermal catalyst temperature range over the entire length of the reaction zone to obtain consistent high yield and lengthened catalyst life. Also, consistent high product yield requires a process with consistent process parameters.
U.S. Pat. No. 5,117,007 discloses that the beneficial effect of adding a phosphorus compound in water, in certain ratios to each other, to control the reaction temperature profile in oxidation of n-butane to maleic anhydride can be obtained over the entire reaction zone of a fixed bed reactor. The beneficial effect occurs over the entire reaction zone including the so-called hot spot temperature zone but also a significant beneficial effect of the reaction temperature profile has been found to result despite operation in the flammability zone which exists wherein concentration of n-butane in the feed is about 1.7 mole %, or higher, and air is the source of oxygen. The required ratio of water to phosphorus in the phosphorus compound relates to the concentration of n-butane, and also to the reactor size and shape. An isothermal reaction zone temperature thereby results wherein the reaction zone temperature gradient is within a maximum range of about 45° C. (80° F.) with consequent increase in overall yield.
As discussed above, one method of phosphorus addition technology is based on adding phosphorus, in the vapor phase, to a fixed bed catalysts or a fluid bed catalyst. In one method of phosphorous addition, for example, phosphorus, in the form of triethylphosphate (TEP), is added in the vapor phase. This technology has provided the beneficial effect of stabilizing and preventing loss of catalyst yield; however, an undesirable effect of adding phosphorus to a VPO fluid bed catalyst used to oxidize butane to make maleic acid or maleic anhydride has been an increase in operating temperature. This high temperature causes undesirable effects, such as limiting the throughput to the reactor and also accelerating degradation of catalyst physical properties such as surface area and pore volume, which are important parameters for good catalyst performance. Therefore, there is a need to find a way to add phosphorus to the butane oxidation reaction to increase maleic anhydride yield without causing an increase in the operating temperature which will harm the catalyst.
We discovered that by impregnating a VPO catalyst with an alkyl ester of an orthophosphate such as triethylphosphate (TEP), the TEP-impregnated VPO catalyst could be used for adding phosphorus to the fluid bed butane oxidation reaction, and thereby achieving the desired increase in maleic anhydride yield at a lower operating temperature and, in addition, at a significantly lower concentration of TEP when compared to the to the vapor phase TEP addition mode.
Thus, the present invention has the advantages of permitting the addition of phosphorus without causing temperature increases which can harm the catalyst and also enabling one to use less TEP to replace lost phosphorus, which provides an economic benefit.